1. Field of the Invention
This invention relates to a heatable transparency, and more particularly, to arranging ends of bus bars relative to an electrically conductive member of a heatable automotive transparency e.g. a laminated windshield to minimize if not eliminate hot spots at the end portions of the bus bars.
2. Discussion of the Technology
Automotive heatable windshields, e.g. of the type disclosed in U.S. Pat. No. 4,820,902 include two glass sheets laminated together by a plastic interlayer, usually a sheet of polyvinyl butyral (xe2x80x9cPVBxe2x80x9d). A pair of spaced bus bars between the glass sheets is in electrical contact with an electrically conductive member, e.g. a sputtered electrically conductive coating of the type disclosed in European Patent Application No. 00939609.4 or a plurality of electrically conductive filaments of the type disclosed in U.S. Pat. No. 5,182,431. Each of the bus bars is electrically accessible by an external lead to pass current from a power source through the bus bars and electrically conductive member to heat the conductive member and thereafter heat the inner and outer surfaces of the windshield by conduction. The heated windshield surfaces attain a temperature sufficient to remove fog, and melt snow and ice. As can be appreciated, heatable windshields are practical, and in some geographical areas are a requirement, during the winter season.
The conductive member is usually on the no. 3 surface of the windshield, i.e. the outer surface of the inner glass sheet of the laminate, although laminated windshields have been made with the conductive coating on the no. 2 surface, i.e. the inner surface of the outer glass sheet. The bus bars may be provided by silk-screening a ceramic paste having metal particles onto the conductive coating, e.g. as discussed in U.S. Pat. Nos. 4,725,710 and 5,128,513, or by positioning a metal foil between the interlayer sheet and the conductive coating as discussed in U.S. Pat. Nos. 5,418,025; 5,466,911 and 5,850,070.
One limitation of the presently available heatable windshields is hot spots at the end portions of the bus bars. Depending on the severity of the hot spots, fractures in the glass sheets may occur. Under certain conditions, these fractures can propagate into the vision area of the windshield requiring windshield repair or replacement. To eliminate hot spots in bus bars or more evenly distribute the current along the bus bar, various techniques have been employed. More particularly, U.S. Pat. No. 5,128,513 discusses varying the width of the bus bars and extending one bus bar laterally beyond the corresponding end of the opposite bus bar.
Other types of available bus bar arrangements are discussed in U.S. Pat. Nos. 3,789,191; 3,789,192; 3,790,752; 3,794,809; 4,543,466; 5,182,431 and 5,213,828.
As can be appreciated, it would be advantages to provide a heatable transparency, e.g. a laminated automotive windshield that minimizes if not eliminates hot spots in bus bars and more particularly in the end portions of the bus bars.
One embodiment of the invention relates to an article having a member responsive to electric stimulation, e.g. a sputter electrically conductive coating or a coating that changes transmittance as a function of applied current. The article includes a substrate having a major surface and peripheral edges; the electrically conductive member is on the major surface of the substrate. The electrically conductive member, e.g. an electrically conductive coating is defined by a perimeter with the perimeter of the electrically conductive coating selectively spaced from the peripheral edges of the substrate to provide an electrically insulated area or non-conductive strip. A pair of spaced bus bars is in electrical contact with the coating. Each of the bus bars has a pair of opposite end portions with at least one of the end portions of one of the bus bars extending beyond the perimeter of the electrically conductive member into the non-conductive strip to minimize, if not eliminate, hot spots at the at the least one end portion. The article incorporating features of the invention may be used to make multiple glazed units, heatable vehicular transparencies and heatable windows for refrigerator doors, to name a few products that may be made using the article incorporating features of the invention.
In another embodiment of the invention, the article is a laminated article such as a laminated transparency, e.g. an automotive windshield having a pair of glass sheets laminated together by a plastic sheet or interlayer with the electrically conductive coating, e.g. a conductive film between a pair of dielectric films and the pair of bus bars between the glass sheets. Ends of the bus bars extend beyond the perimeter of the conductive coating into the non-conductive strip to minimize if not eliminate hot spots at the end portions of the bus bars and preferably terminate short of the peripheral edge of the glass sheet. A lead is connected to each of the bus bars and extends from its respective bus bar beyond the peripheral edge of the windshield to provide electrical access to the bus bars and the conductive coating.
Additional features of the invention to minimize, if not eliminate, hot spots at the end portions of bus bars include the following. A pair of bus bars having different lengths has one of the bus bar extending along the topside of the conductive coating and the other one of the bus bars extending along the bottom side of the conductive coating. The portions of the coating between the bus bars do not extend beyond the ends of the longer bus bar. In still a further feature of the invention, the windshield has a vision area having a top edge and a bottom edge. The xe2x80x9cvision areaxe2x80x9d is defined as the see through area of the windshield available to the driver and/or passenger. The coating has a top edge beyond or adjacent the top edge of the vision area and a bottom edge below or adjacent the bottom edge of the vision area. The top bus bar is adjacent the top edge of the coating and the bottom bus bar is adjacent the bottom edge of the coating with the bus bars outside the vision area. The bottom edge of the coating is spaced a greater distance from the bottom edge of the vision area than the bottom bus bar, and the top edge of the coating is spaced a greater distance from the top edge of the vision area than the top bus bar.
In still a further embodiment of the invention, the bus bars and leads are contiguous with one another and are each a metal foil. The bus bars are adhered to the plastic interlayer of the windshield, and the leads extend beyond the edge of the windshield to provide external electric access to the bus bars. An air barrier arrangement of the type disclosed in U.S. patent application Ser. No. 10/201,863, filed even date is provided around the leads to prevent the ingress of air between the lead and the glass sheet having the conductive coating after edge sealing and during autoclaving.
The invention further relates to a method of making the laminate.